1. Field of the Invention
The present invention relates generally to apparatus for applying photosensitive resist to precision chemically machined components and more particularly to an apparatus for spraying photoresist on sheets of chemically machined components.
2. Description of the Prior Art
Photoresist has been used extensively in the process of chemically machining precision metal components. The photoresist process is basically a three step process. First, a coating of resist is uniformly applied over a metallic sheet of appropriate thickness. Second, this sheet is sandwiched between a pair of photomasks and exposed to actinic light. Finally, the sheet is developed by an appropriate developing process.
After developing, the non-developed areas of the photoresist are removed by using an appropriate solvent. The sheet is then dried and immersed in a chemical bath. The chemical bath may be either acidic or basic in nature, and attacks those portions of the metallic sheet which are not covered by resist. The developed photoresist serves to ward off the solution and prevent it from attacking those portions of the metallic sheet which are covered.
After the initial etching step, it is usually desirable to selectively plate a second metal over certain portions of the first metal, e.g., gold plating certain portions of integrated circuit lead-frames. In order to accomplish this, a second photoresist application is utilized. Photoresist is applied to the etched metallic sheet; the sheet is sandwiched between photomasks and developed; and the non-developed portions are removed by a suitable solvent as in the etching process. The sheet is then immersed in a metallic plating chamber and plating occurs on those portions of the etched sheet which are not covered by photoresist.
In both photoresist processes described above, it is important that the photoresist be applied evenly and uniformly over the entire surface of the sheet. Moreover, with respect to selective metal plating, it is important that the photoresist cover the etched edges of the sheet as well as the planar portions to prevent plating from occurring where it is not desired. Photoresist has been applied in the past by using roller coating techniques wherein photoresist is rolled across the sheets similar to the method in which paint is applied to household walls.
This method has been found to be unsatisfactory in that the coating realized varies in thickness and coverage and oftentimes results in overcovered or undercovered areas. These coverage variations result in imprecise images and reduce the overall quality of the plated component.
Moreover, roller techniques may not adequately protect the edges formed during the initial etching process which allows plating metals to adhere to the etched edges. This increases the expense of selective plating in that more precious metal plating is used than is necessary.